1. Field of the Invention
The present invention relates to a semiconductor structure and more particularly to a structure comprising diffused integrated resistors and to an application of such structure to an amplification circuit.
2. Discussion of the Related Art
FIG. 1 illustrates an electronic amplification circuit 1, currently called “current sense amplifier”, enabling measuring the current flowing through a line. Amplification circuit 1 comprises two inputs 2 and 3 and one output O. First input 2 is connected to the first terminal of a resistor R1. The second terminal of resistor R1 is connected to the positive input of an operational amplifier (OA) 4. Second input 3 is connected to the first terminal of a resistor R2. The second terminal of resistor R2 is connected to the negative terminal of operational amplifier 4. The positive input of operational amplifier 4 is also connected to the collector of an NPN transistor 5 having its base connected to the output of operational amplifier 4. The emitter of transistor 5 is connected to an element 7 of amplification of gain K, and the second terminal of element 7 is grounded via a resistor RL and is connected to output O of amplification circuit 1 via an amplifier with a high input impedance (B) 9. The differential input voltage of amplification circuit 1, taken between input terminals 2 and 3, is called VRS, and the output voltage of the amplification circuit, taken from output O, is called VO. The average voltage of input terminals 2 and 3 with respect to ground is called the common-mode voltage, VCM. A resistor RS is placed outside of amplification circuit 1, between input terminals 2 and 3. Resistor RS is placed on a line (not shown), the current of which is desired to be measured.
In known fashion, in the case where resistors R1 and R2 are equal, amplification circuit 1 illustrated in FIG. 1 has a gain G equal to:
  G  =                    V        O                    V        RS              =          K      ⁢                        R          L                          R          1                    
The values of resistances R1, R2, and RL and of the ratios between these resistances must thus be very accurate since they set the gain of amplification circuit 1.
The case where resistors R1, R2, and RL are diffused integrated resistors is here considered. Such resistors have the advantage of being able to be formed at low cost and of having a relatively accurate value.
FIG. 2 illustrates a conventional embodiment of a diffused resistor. A lightly-doped N-type semiconductor layer 23 is formed on a lightly-doped P-type semiconductor substrate 21. In layer 23, an N-type well 25 is delimited by a P-type wall 27 which extends down to substrate 21 and which is connected to a reference voltage, for example, to ground, to isolate well 25 from the other components formed in and on semiconductor layer 23. A portion 29 of well 25, in which the resistor is formed, is delimited, vertically, by a heavily-doped N-type buried layer 31, at the limit between layer 23 and substrate 21 and, horizontally, by a heavily-doped N-type wall 33 joining buried area 31. In the upper portion of well 25, 29, a P-type doped region 35 forms the resistor. This region 35 is, for example, a rectilinear region having its length-to-width ratio setting the desired resistance value. Two connection terminals 37 and 39 are taken at two locations of region 35.
In the case where this resistor is used as resistor R1 of the amplification circuit of FIG. 1, terminal 37 is connected to input 2 of amplification circuit 1 and terminal 39 is connected to the internal components of circuit 1.
There are two conventional ways of biasing well 25, 29. The first one comprises biasing well 25, 29 to a voltage V1 (as shown in FIG. 2) and the second one comprises letting well 25, 29 float.
In the first case, a voltage V1 is applied on wall 33 and thus on buried area 31, which uniformly biases well 25, 29. Voltage V1 must be greater than the maximum voltage applied to the resistor to avoid for the PN junction, between P-type region 35 and N-type well 25, 29, to be forward biased.
However, the biasing of the well to a voltage V1 raises two problems, in particular when the resistor is used in an amplification circuit such as that of FIG. 1, in which the common-mode voltage is likely to vary strongly. The first problem is of course that PN junction 35-29 risks being forward biased. The second problem is that the extension of the space charge area in P-type region 35 depends on the voltage difference between voltage V1 and the common-mode voltage. A variation in the common-mode voltage thus causes a variation in the value of the resistance between terminals 37 and 39. In the expression of gain G of the amplification circuit of FIG. 1, it should be noted that a simultaneous variation in the values of resistors R1 and R2 has no influence upon the gain, and thus causes no problem. However, a variation in the value of resistance R1 modifies the gain value since the value of resistance RL remains constant. Indeed, resistance RL is not submitted to the voltage variations at the input of the amplification circuit. A variation in gain G is generally not desired.
In the second case, well 25, 29 is left floating. Thereby, the voltage of floating well 25, 29 is equal to the highest voltage in region 35, minus approximately 0.6 V corresponding to the forward voltage drop of a PN junction. The extension of the space charge area in P region 35 is then very small and substantially constant.
In an alternative to the second case, well 25, 29 could be biased to the highest voltage of the resistor by connecting wall 33 to terminal 37. However, if the voltage applied at input 2 of the circuit, and then at terminal 37 of the resistor, becomes negative, as is often the case in automotive applications, the PN junction formed by the P-type wall 27 connected to ground and the N-type well 25, 29 can become forward biased. This increases the risk of parasitic transistors or thyristors being formed.
The second case (floating well 25, 29) appears to be the most interesting. It has thus been attempted to form diffused resistors placed in floating wells. Structures of amplification circuits measuring a current comprising such resistors have been tested by the present inventors, and the latter have found that, for a constant current in resistor RS and thus a constant value of VRS, output voltage VO of the amplification circuit varies when the average voltage on the resistor varies. In other words, the amplification circuit gain varies when the input terminals are submitted to voltage fluctuations.